In an extracorporeal circulation therapy in which blood is pulled out of the body of a patient for an extracorporeal treatment by a blood processing apparatus and the blood is returned into the body after the treatment, usually, a pressure sensor is provided to measure a pressure in the extracorporeal circulating circuit. As an example of the means for measuring a pressure in an extracorporeal circulating circuit, Patent Document 1 describes a pressure measuring method using a drip chamber which is commonly used in an extracorporeal circulation therapy.
FIG. 33 is a schematic configuration view showing an example of a pressure measuring method using a drip chamber. As shown in FIG. 33, a drip chamber 2 is disposed in the middle of a liquid flow path 8, and is configured with a branch tube 500 branched from the top of the drip chamber 2 and a liquid chamber pressure measuring means 61 at the end of the branch tube 500. In a pressure measuring method using such drip chamber as shown in FIG. 33, a certain volume of the drip chamber 2, e.g. a half of the volume of the drip chamber 2, body fluid or medicinal solution is stored in the drip chamber 2 with the remaining half of the volume being filled with a layer of air to perform an extracorporeal circulation therapy. The means for measuring pressure of an air chamber measures a pressure in the liquid flow path 8 without a directly contact with the body fluid or medicinal solution due to the air layer.
However, the drip chamber 2 has an inner diameter which provides a large contact area between the body fluid or medicinal solution and the air, and further provides a large volume of the body fluid or medicinal solution for storage. Thus, it takes a long time to exchange the stored liquid with a liquid to be newly introduced, which may cause retention or coagulation of the body fluid or medicinal solution.
As an example of a pressure sensor to solve the above problem, Patent Document 2 describes a pressure measuring method for measuring a pressure in a liquid flow path via a deformable plane (a deformable portion which is deformed by a pressure in an extracorporeal circulating circuit) as a pressure measuring method to avoid the contact between body fluid or medicinal solution and air.
FIG. 34 is a schematic view showing an example of a pressure measuring method for measuring a pressure in an extracorporeal circulating circuit via a deformable plane. As shown in FIG. 34, a pressure sensor 3 in the prior art is disposed on the way to a liquid flow path 8, and measures a pressure in a liquid chamber 6 by detecting a deformation quantity of a deformable plane 20 which is at least partially deformed by a pressure in the liquid chamber. In FIG. 34, the elements having the same function as those in FIG. 33 are given by the same reference numerals as those in FIG. 33.
In the configuration of the pressure measuring method shown in FIG. 34, the pressure sensor 3 in the prior art includes a liquid flow inlet 40 and a liquid flow outlet 41 which are substantially located in-line. When a liquid is introduced in the liquid flow inlet 40 to be flown into the liquid chamber 6, the flow path is suddenly widened at the exit of the liquid flow inlet 40, and therefore the convection is generated at the liquid flow inlet 40 to cause the liquid flow stagnates. As a result, since the body fluid or medicinal solution remains at a certain position, coagulation of the body fluid may occur.
In the case of a low flow rate, no turbulence is generated in the flow in the liquid chamber 6. However, in this case, since the introduced liquid goes to the liquid flow outlet 41 which is substantially in-line with regard to the liquid flow inlet 40, the exchange of the liquids in the liquid chamber 6 is not promoted, which may result in coagulation of body fluid therein. In addition, the pressure in the pressure sensor 3 in the prior art as shown in FIG. 34 is highly variable, and in the case of a negative pressure, the deformable plane 20 closely contacts a wall surface of the liquid chamber 6, and therefore the liquid flow outlet or the liquid flow inlet may be blocked. In this case, since the flow of body fluid is stopped, coagulation of the body fluid may be caused.
Also, because the deformable plane 20 has a corrugated shape, the air chamber 9 has to have a sufficient depth (which at least has size of not less than corrugated shape) to a certain degree in the direction perpendicular to the direction in which the deformable plane 20 is disposed to give a margin for the width of the corrugated shape in the direction of its convexo-concave configuration. This does not allow the air chamber 9 to have a smaller volume. Thus, in measuring a negative pressure, the deformation quantity of the deformable plane 20 in the direction toward the liquid chamber 6 is increased, which eventually increases the volume of the liquid chamber 6 and easily causes the above described stagnation.
Furthermore, the deformable plane 20 in the pressure sensor 3 in the prior art as shown in FIG. 34 may be damaged due to the soft material thereof. In case of damage of the deformable plane, the operation is just like the pressure measuring method using the drip chamber shown in FIG. 33, and cannot avoid the above described problems of coagulation due to the contact between the air and the body fluid or medicinal solution.
Moreover, in the pressure sensor 3 in the prior art as shown in FIG. 34, when the deformable plane 20 is deformed, the pressure in the air chamber 9 changes in correlation with the pressure in the liquid chamber 6. This causes the differences between the pressure characteristics obtained in the case where a pressure is measured via air and in the case where a pressure is measured via the deformable plane, thereby resulting in a problem that no pressure can be correctly measured.
In addition, the pressure sensor 3 in the prior art as shown in FIG. 34 is a disposable product which can be discarded after use, and this requires the connection between a pressure sensor and a pressure measuring means every time the pressure sensor is used. Therefore, if there is any incomplete connection, the leakage between the pressure sensor and the pressure measuring means is caused, thereby making it impossible to correctly measure a pressure therein. Since the leakage provides the air chamber side with an infinite volume, the deformable plane 20 is significantly deformed toward the liquid chamber when the liquid flow path 8 has a negative pressure. As a result, the deformable plane 20 blocks the liquid flow inlet 40 or liquid flow outlet 41, the flow of body fluid or medicinal solution is stopped, and therefore eventually may cause coagulation of the body fluid.
Patent Document 3 describes a pressure sensor for stably measuring a pressure by automatically changing the volume of air on an air chamber 9 side in conjunction with the pressure on a liquid chamber 6 side, so as to control a position of the deformable plane 20.
FIG. 35 is a schematic view showing an example of the configurations of a hydraulic measuring apparatus. As shown in FIG. 35, the pressure sensor 3 in the prior art is configured with, in addition to those of the pressure sensor shown in FIG. 34, a communication section 51 for controlling the volume of air in the air chamber 9, a pump 400 disposed on the communication section 51, a valve 401, air chamber pressure measuring means 60, and second pressure measuring means 62. In FIG. 35, the elements having the same function as those in FIG. 34 are designated by the same reference numerals as those in FIG. 34.
However, the hydraulic measuring apparatus shown in FIG. 35 needs to have a pump, a valve, and separate pressure measuring means mounted thereto, in addition to a pressure sensor which measures a pressure, which inevitably makes the configuration of the apparatus complicated and in turn causes an increased cost of the apparatus. Furthermore, in order to perform a stable pressure measurement, the volume of air in the air chamber should be strictly controlled, which causes the problem that the control requires tremendous accuracy.    Patent Document 1: JP-A-2002-282355    Patent Document 2: JP-A-09-024026    Patent Document 3: JP-A-08-117332